VMFS extent line in description file should be with 4 fields:
RW <size> VMFS "file-name.vmdk"
Check the number explicitly and report error if offset is appended as
FLAT, which should be invalid format.
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>